Without purchasing expensive air-con units, or solar panels - and the associated moutings and electrical paraphernalia, is it possible to create a temperature difference of 10-20 °C using materials one can purchase for less than $1000?

Say you have a glass-roofed courtyard (or conservatory of some kind) and the average outside air temperature in summer is between 30-40 °C. With direct sunlight on the glass roof the internal air temperature could easily reach 50-60 °C. Not a very comfortable environment - I'm sure you'll agree.

I would like to design and build some relatively inexpensive system to maintain the internal air temperature to around 25 °C. With the desert-like conditions outside, there is certainly plenty of solar energy available. In fact, before I buried it, water in the black hosepipe that supplied the shower used to reach scoldingly-hot temperatures (say about 70 °C for example.)

I envisage the solution might involve some arrangement of black water-filled pipes, two sealed reservoirs at different heights, a heat pump, a generator and...??? To be honest I really don't have a clue how I could set this up so as to create the desired temperature difference.

It may also be helpful to note that near the building there is an escarpment with a steep drop of about 20m. I've heard that storing energy by pumping water to a greater height is particularly efficient. I doubt however that the same could be said about a cheap turbine/generator used to generate electrical power as the water descends.

If the most efficient/inexpensive solution happened to involve generating a surplus of electrical power - then this could be utilised in the house and would certainly be an advantage.

(Perhaps I should explain - my parents are planning to retire to their small holiday-home in Sicily next year. I'm hoping to build a temperature-controlled courtyard for them as a surprise retirement gift. Also, a water feature incorporating laminal valve jets of water and a Tesla fountain!)

This question came from our site for active researchers, academics and students of physics.

@David: I used TeX in the question; surely that should persuade you to let it stay here ;-)
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qftmeJun 9 '11 at 19:52

I actually like the suggestion to send this to DIY (I just never remember it's there), the only thing you're getting from LaTeX is a script font and the "degree" symbol. Not that it is a bad question, just that the physics content is pretty superficial.
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dmckeeJun 9 '11 at 21:03

@qftme: hehe ;-) it doesn't quite work that way.
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David ZJun 9 '11 at 21:04

@David amd@dmckee I think it has as much physics as "climate science". Applied physical principles.
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anna vJun 10 '11 at 12:43

4 Answers
4

1.) Have you thought of so-called "swamp coolers" (evaporative coolers)?

These devices work on the principle of evaporative cooling. There are 2 basic types: Direct cooling and indirect cooling. The direct cooling units are VERY simple and easy, but may result in air that is too humid to be comfortable (even though it is much cooler). The indirect is more expensive and hard (but not nearly as expensive as conventional AC), and results in drier air. Some systems use both methods: pre-cool the air via the indirect method, then run the cooled air through a direct cooler. The result is cooler and drier air than either method alone. All methods work best with very dry air (which it appears you have).

DIRECT method: Dry outside (or even inside) air is drawn through a wet screen or filter. The cooled, moist air is pumped directly inside. This method can be as simple as throwing a wet towel over a fan.

INDIRECT method: Dry outside air runs through a wet screen or filter, to get cooled, and then across a heat exchanger which. The dry air picks up moisture which cools the unit. Inside air blown into the opposite chamber of the heat exchanger is cooled, but picks up no moisture. Even this relatively complicated method is easier and very much cheaper to build (and operate) than an AC unit.

2.) The simplest way to keep things cool is to deny the sun entrance.

I had a home with a large skylight in the kitchen. Because I lived in a very temperate part of the northwestern U.S., I had no AC. In the summer I would block off the skylight totally with a very light custom-cut board lined with aluminum foil; then take it down in the fall. It made a huge difference (probably 5C in the kitchen).

You could do something similar, using greenhouse shade cloth, chimney flashing, or many other items that run the gamut of the aesthetics/functionality tradeoff.

3.) Venting the hottest air (from the top) and replacing it with cooler air will help; or you could leave that air alone and recirc the cooler lower air, to make it even cooler. In either case, forcing the air through deeply buried pipes would cool it.

The temperature of the earth, almost anywhere on Earth you would want to live, is roughly in the neighborhood of 50F (10C). You just have to dig down deep enough to access that temperature. All the desert critters know this: that is why they stay underground during the day. Even if you only want to go down two meters, you should be able to access constant temperatures of lower than 15C. Then you bury some pipes and force the air through them. The air heats the dirt, but that dirt is cooled by the surrounding dirt. Basically you are using mother earth as a heat sink.

Yeah for swamp coolers. We used to get 10--15 degrees Celsius separation with these things in a Las Cruces, NM summer. 'Course, on the three or four humid days a year they were worthless. And you pay in terms of noise, too: those big fans make some.
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dmckeeJun 9 '11 at 17:23

Good suggestions. One quibble the temp a few meters down is not 10C, it is very similar to the average annual temperature of the location. I bet in Sicly it is well above 10C, but still cooler than summertime temps. I use a prtable swamp cooler (you have to feed it with a bucket) to reduce my AC demands in summer (California). You can go with a hybrid solution if that works better for you.
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Omega CentauriJun 9 '11 at 18:36

@Vintage: Many thanks for your suggestions. I've been going over the wiki pages on swamp coolers (which I'd never heard of before) and will definitely try to impliment something similar. WRT blocking out the sun altogether, I want to avoid anything cumborsome to install / remove as the view of the stars at night is amazing! To which end I've also been looking at different methods to achieve 'smart glass' (variable transparency). Stuggling to find a cost-cutting shortcut to achieving this at the moment :S
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qftmeJun 9 '11 at 19:59

qftme: Full on sunlight delivers roughly a thousand watts per square meter. Multiply by the area of your enclosure and you can come up with a very large number. So you'll almost certainly have to find a way to exclude most of it. They do sell reflective films that can be added to glass. Either silvered films which reflect 80-90% of the incoming energy, or some that only reflect in the infrared so they aren't very noticable, but then they only reflect 40-50% of the incoming energy.
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Omega CentauriJun 9 '11 at 22:16

@Omega Centauri note he is talking of Sicily. During the hot months, may through september the sun is quite high and an extension of the roof of about 2 meters and/or cloth sun shades for eastern and western exposures outside eliminate most of the direct heat.
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anna vJun 10 '11 at 6:11

I have been reading on geothermal solutions. These take advantage that once you are a few meters below the ground surface the temperature is more or less constant,maybe 10-18C. There are solutions sold which circulate water below ground and bring it in , but with a heat pump in the system, to cool houses in the summer and heat them in the winter (18C is nice heat in the winter).

You mention Sicily. Is there a well in the plot? Wells in Greece are over 10 meters in depth and thus in the the cool range of temperatures, and often the water is quite deep. We used to lower watermelons in the water to cool them. A circulating system within the water of the well might cool a room with only the expense of the pump and the pipes and maybe a small fan to circulate the air. One would have to calculate to see if the system can be efficient enough without a heat pump.

You might also look into air wells in combination with the geothermal suggestion.

Have fun.

Edit: I would like to add that outside extended sun shades are very practical at the latitude of Sicily. They can be pulled in at night, when temperatures fall and one opens all windows for good circulation and heat exchange the traditional way.

Another suggestion, more long term, is to plant deciduous trees with thick leaves in the summer to shade the house all around. The mimosa of Costantinople is very good at this. It grows fast and shades in umbrella shape in the summer and at night its leaves fold so the night air is not impeded in circulating around the house. Note that trees lower the temperature by 2 degrees also. In the winter there are only branches and do not obstruct sunlight to reach and heat the house.

@Anna: Thank you. Your suggestions look really interesting. So much so in fact that I've just emailed the architect (my borther [not a real architect!]) with the suggestions - that a 'solar chimney' and a 'Trombe wall' be factored into the designs. I also checked with my Dad about the well - he said it's probably better described as a sump, but it does at least go $2-3m$ underground. The trouble is that it's at the bottom of the escarpment, $20m$ lower than the house. Where the house is (at the top) the soil is only $50cm$ deep before you get to solid rock.
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qftmeJun 9 '11 at 20:08

It's nowhere near a complete solution, but I think you would get a large impact by adding some kind of vent to the top of your roof, allowing the high-temperature air to escape, rather than build up. This might also provide for some inward airflow, if you could arrange that to pull in (hopefully cooler) air from outside.

If the outside air is too hot, but is dry, then maybe you could let the inward air flow pass over an evaporative cooler (even a damp towel) on its way in. Evaporative coolers are very powerful, in dry areas, if the water supply is not a problem.